1. Field of the Invention
The present invention relates to a projection-type image display apparatus such as a rear-projection-type television or a video projector that includes a reflection-type image forming element or a transmission-type image forming element.
2. Description of the Related Art
To improve the image quality of a projection-type image display apparatus, it is necessary to achieve high positional accuracy in the arrangement of optical elements such as a lens and a mirror that constitute a projection optical system in the apparatus. It is also important to reduce a deformation in the optical elements caused by thermal expansion or thermal contraction. Particularly, when the optical length between adjacent optical elements in a projection optical system is short, as in the projection-type image display apparatuses, position misalignment or thermal deformation of the optical elements significantly affects the image quality. That is why it is necessary to enhance the positional accuracy and control the thermal deformation of the optical elements.
For example, Japanese Patent No. 3643810 discloses a mirror retention mechanism that is used in retaining a resin mirror in a rear-projection-type graphic display apparatus. In the mirror retention mechanism, a supporting frame supports the resin mirror so that the resin mirror can radially expand or contract with respect to the central portion of its bottom end. Moreover, to reduce the thermal deformation of the resin mirror, a rear-side member is arranged at a distance behind the supporting frame and biasing members are used to elastically bias the resin mirror in an anterior direction of the rear-side member.
To retain a resin mirror at high positional accuracy by using the abovementioned mirror retention mechanism, it is necessary to achieve high manufacturing accuracy for the supporting frame and high assembly accuracy for the mirror retention mechanism. Thus, such a configuration causes an increase in the manufacturing cost of a projection-type image display apparatus, such as a rear-projection-type graphic display apparatus.